xtensa: fix hard-coded interrupt value for PS register

There is a hard-coded value of PS_INTLEVEL(15) to set the PS
register. The correct way is actually to use XCHAL_EXCM_LEVEL
with PS_INTLEVEL() to setup the register. So fix it.

Fixes #31858

Signed-off-by: Daniel Leung <daniel.leung@intel.com>